Halftoning is the process of representing a continuous tone image by a bi-level image such that, when viewed from a suitable distance, the bi-level image gives the same impression as the contone image. Halftoning often employs a halftone screen. A halftone screen describes the set of values which together make up the set of thresholds to be applied in a halftone screening process to generate the output halftone patterns. A pattern of dots used to produce a particular shade of color is known as a halftone screen. A single-center halftone screen uses the entire area for one cell, or tile, only. The resolution of a halftone screen is defined by the number of lines of dots in one inch, measured parallel with the angle of the halftone screen. The higher the resolution of the halftone screen, the greater the detail that can be reproduced. Halftone screens use a matrix of threshold values. A halftone screen can be a data set with different print density values equally represented or with a controlled unequal distribution for gamma-compensated screens. For monochrome printing, the image data is then compared with the screen thresholds at each position. If the image data exceeds the threshold, a dot is printed. Otherwise, that particular location remains unprinted.
Halftoning techniques are widely employed in the printing of digital images. The general idea behind halftoning is that, by varying the density of the dots used to print the individual primary colors, Cyan, Magenta, Yellow and Black (CMYK), any shade can be reproduced. By varying dot density, the eye perceives a shade somewhere between the solid color and the color of the background paper. The effect has its limits. When the dots get too small or are spaced too far apart, the eye starts perceiving individual dots again and the image looks grainy.
For printing with multiple overlapping colors, halftoning presents particular challenges. For dot-on-dot printing, single halftoning screens tend to be used. This has the disadvantage of reducing a spatial frequency with respect to a distribution of dots printed at different locations. This tends to produce an image which appears grainy. The same can be said for clustered dot printing in which different color dots are printed adjacent to each other to create a multi-dot cluster that is visually perceived as some desired intermediate color.
For multi-function device applications, color error diffusion is a popular halftoning method due to its detail preservation and moiréresistance. Vector error diffusion is one color error diffusion method which produces good image quality because it achieves higher halftone quality compared to other channel-independent error diffusion methods. Methods for hierarchical error diffusion help control dot distribution for both primary and secondary color dot formation which covers the class of error diffusion that follow telescopic dot firing constraints. One key element in vector halftoning and hierarchical error diffusion is the CMYK to CMYKRGB color transform that helps controls the dot-overlapping strategy.
Accordingly, what is needed in this art are increasingly sophisticated methods for performing a CMYK to CMYKKpRBG transform to help reduce engine-specific halftone artifacts in printing devices capable of dot-on-dot printing.